1. Field of the Invention
The present invention relates to a drill that partly has four cutting edges.
2. Description of the Related Art
A drill is generally used as a boring tool. A two-edge drill is frequently used (for example, see Japanese Unexamined Patent Application Publication No. 2008-36759). The two-edge drill has a relatively small number of cutting edges, and hence the two-edge drill can have large grooves. Thus, machining dust is efficiently discharged. As long as the machining dust is efficiently discharged, the feed speed of the drill can be increased, and the boring time with the drill can be decreased. However, since the two-edge drill has the relatively small number of cutting edges, wear of the cutting edges may progress relatively quickly.
FIG. 7 is a cross-sectional view of a typical four-edge drill. The four-edge drill has a larger number of cutting edges than the two-edge drill. Accordingly, as shown in FIG. 7, grooves at the front of the cutting edges in the rotation direction cannot be set large and the discharging efficiency for machining dust may be degraded. Thus, it may be difficult to increase the feed speed of the drill and to decrease the boring time with the drill. On the other hand, since the four-edge drill has a relatively large number of cutting edges, wear of the cutting edges progresses relatively slowly, and holes can be finished with high quality for a long term.